Existing processes for the production of linear alpha olefins (LAOs), including comonomer-grade 1-butene, 1-hexene, and 1-octene, rely on the oligomerization of ethylene, and lead to a mixture of ethylene oligomers having a chain length of 4, 6, 8, and so on. Without being bound by specific theory, this is due to a chemical mechanism mainly governed by competing chain growth and displacement reaction steps, leading to a Schulz-Flory- or Poisson-product distribution. From a commercial standpoint this product distribution poses a formidable challenge for the full-range linear alpha olefins producer. The reason is that each served market segment exhibits a very different behavior in terms of market size and growth, geography, fragmentation etc. It is, therefore, very difficult for the LAO producer to adapt to the market requirements since part of the product spectrum might be in high demand in a given economic context, while at the same time other product fractions might not be marketable at all or only in a marginal niche.
LAOs such as 1-butene, 1-hexene, and 1-octene are suitable for the production of polyethylene, including linear low density polyethylene (LLDPE). Currently, the main LAO used in the production of polyethylene is 1-butene, followed by 1-hexene. Demand for the latter has increased, which can be attributed to the superior properties of polyethylene made from 1-hexene. At the same time, certain grades of polyethylene materials call for improved physical properties such as superior tensile strength and crack resistance, requiring the presence of 1-octene.
Oligomerization of ethylene usually proceeds in the presence of suitable catalysts. Several of the existing ethylene oligomerization, i.e., dimerization, trimerization or tetramerization, catalysts have one or more disadvantages. These shortcomings, which can affect the corresponding processes wherein these catalysts are used, include one or more of low selectivity for the desirable products, i.e., 1-hexene or 1-octene (due to formation of undesired byproducts from side reactions); low purity of the products themselves, i.e., low selectivities for the LAO isomer within a specific C6- or C8-cut (isomerization, branched olefin formation etc.); wax formation, i.e., formation of heavy, long-chain (high carbon-number) products; polymer formation (polyethylene, including branched and/or cross-linked PE), which this may lead to considerable LAO product yield loss as well as fouling of equipment; poor turnover rates/catalyst activity, resulting in increased cost per kg product; high catalyst- or ligand cost; complex, multi-step ligand synthesis, resulting in poor catalyst availability and high catalyst cost; susceptibility of catalyst performance, both in terms of both activity and selectivity, to trace impurities (leading to, for example, catalyst losses/poisoning); difficult handling of catalyst components in a technical/commercial environment (e.g., during catalyst complex synthesis, pre-mixing, inertization, catalyst recovery, or ligand recovery); harsh reaction conditions, i.e., high temperatures and pressure, resulting in a need for special equipment (increased investment-, maintenance-, and energy costs); high co-catalyst/activator cost or consumption; susceptibility to varying co-catalyst qualities, which is often the case when larger amounts of relatively ill-defined compounds are used as activators (e.g., certain MAO-varieties).
There accordingly remains a need in the art for catalyst systems for the oligomerization of ethylene that can yield 1-octene or 1-hexene with high selectivity. There remains a further need in the art for catalyst systems that can furnish combinations of 1-octene and 1-hexene. At the same time, there remains a need in the art for catalyst systems that can provide combinations of 1-octene and 1-hexene at a predetermined ratio, wherein the predetermined ratio has been a range of available ratios that the catalyst system also provides. There also remains a need for processes for the oligomerization of ethylene to produce 1-octene and 1-hexene in high purity and at variable ratios.